1. Field of the Invention
The present invention is directed in general to composite digital radio broadcast receivers and methods for operating same. In one aspect, the present invention relates to methods and apparatus for blending digital and analog portions of an audio signal in a radio receiver.
2. Description of the Related Art
Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers using existing radio bands. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital radio broadcasting, transmits digital radio and analog radio broadcast signals simultaneously on the same frequency using digitally modulated subcarriers or sidebands to multiplex digital information on an AM or FM analog modulated carrier signal. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception. With this arrangement, the audio signal can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers by transmitting the analog audio AM or FM backup audio signal (which is delayed by the diversity delay) so that the analog AM or FM backup audio signal can be fed to the audio output when the digital audio signal is absent, unavailable, or degraded. In these situations, the analog audio signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog as the digital signal become unavailable. Similar blending of the digital signal into the output audio signal occurs as the digital signal becomes available by attenuating the analog signal such that the audio is fully blended to digital as the digital signal becomes available.
Notwithstanding the smoothness of the blending function, blend transitions between analog and digital signals can degrade the listening experience when the audio differences between the analog and digital signals are significant. Accordingly, a need exists for improved method and apparatus for processing the digital audio to overcome the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.